1. Field of the Invention
The present invention relates to a method for transferring a batch of wafers from one station to another and to a wafer transfer system utilizing this method, wherein a plurality of wafers are automatically transferred from one processing station to another processing station sequentially, the wafers being processed at each station, and more particularly, to the method by which amount of clean air supplied to a clean room/bench may be reduced as much as possible. The wafer transfer system of the invention is useful when applied to chemical processing equipment which requires exhausting a lot of harmful gas or vapor produced during processing.
2. Description of the Related Art
It is well known that most semiconductor fabrication processes, particularly wafer processes, are performed in a clean room. Further, in order to enhance local cleanliness of the air on a work bench and to exhaust any harmful gas or vapor thereby produced, a clean bench system is often utilized in the clean room.
An exemplary existing wafer transfer system applied for a wet-processing equipment is first described, wherein the wet-processing equipment as a whole forms a clean bench. With a trend toward an increasing amount of wafers for processing, the wet-processing equipment becomes more large-sized, which requires an increased amount of exhaust gas.
FIGS. 1(a) and 1(b) show schematically a front and a side view of a wet-processing equipment 15 of the prior art, the equipment itself forming a clean bench.
A plurality of semiconductor wafers 1 are vertically held in a carrier 2, and the carrier is suspended by a hanger 30. A transfer robot 3 having a horizontal arm 31 supports the hanger 30. The transfer robot 3 together with a center rode 33 can move up-and-down by a mechanism (not shown) housed in a box 35, and further the transfer robot 3 can move in a horizontal direction from left to right as shown in FIG. 1(a) by a transfer mechanism (not shown). Chemical solutions or water 18 are contained in baths 8, each of which has a respective automatic cover 12. When the carrier 2 with wafers 1 moves to a position of each bath, the cover 12 is opened and the wafers are lowered through the opened cover and dipped in the chemical solution or the water 18 for chemical treatment or washing.
The entire processing equipment 15 is installed in a clean room, in which a downward flow of clean air is produced as shown by arrows 39. Air filter 16 of HEPA (High Efficiency Particulate Air-filter) and fan 37 are provided on the top of the equipment 15. The clean air 39 in the clean room is inhaled and spouted out from the air filter 16 and forms a downward air flow 38 in the equipment 15. Gas and/or vapor, briefly called hereinafter chemical vapor 19, produced during the process flows into an exhaust opening 9 and is exhausted through an exhaust duct 7.
The wet-processing equipment presently used have a clean bench type structure as described above, and clean air spouted out from the air filter 16, which occupies nearly 2/3 of a floor space of the equipment, is almost exhausted out of the clean bench. In order to maintain the downward air flow 38 in the equipment 15 and to confine the chemical vapor produced during the processing and transport steps into a limited space, the air flow from the filter 16 on the top of the equipment 15 is required to have a flow velocity substantially equal to or larger than about 0.3-0.4 m/sec.
To maintain high air velocity in the clean bench system of the equipment 15, requires a large amount of the clean air supplied to the clean room itself, and further an amount of the exhaust air which is to be treated and thereafter abandoned becomes large. These factors increase a running cost of the clean room having the wet-processing equipment 15.
In order to enhance cleanliness of limited ambient air around wafers, a wafer carrier integrated with a filter and a fan is known; however, this type of wafer carrier takes no thought of a clean air suppply to the clean room.
Further, Japanese Laid-Open Patents SHO 63-248449 and HEI 2-277556 disclose wet-processing equipment, wherein chemical vapor is prevented from diffusing into the clean room/bench by utilizing a horizontal air curtain above a chemical processing bath. The method also does not take saving or conservation, of the clean air supply to the clean room into consideration.